Can I charge computers by inducing 5-volt into their USB connectors? Is this possible? I noticed that when my EeePC 900 is off but plugged in charger, there is power coming out of the USB port. Can this be a sustainable way of charging a PC, or a flaw in design?

To answer your side question, power comes out of the USB port when the computer is off for the convenience of those devices that use USB for charging (such as an MP3 player). This means that you can charge the external device without having to turn the computer on. As for your primary question, I've never heard of a computer that's charged via USB.
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Michael ToddJan 11 '10 at 5:19

In general, no, it's not possible (for a laptop or similar computer). Not unless the device is specifically designed to charge through it's USB port (and it states so in the unit's documentation) and the appropriate cable is provided.
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Daniel R HicksAug 10 '14 at 13:06

6 Answers
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I have an EEE PC with an additional charger which I can use inside my car. It plugs into the cigarette lighter thingie that seems to be standard in every car. When on the move, it does allow me to charge my laptop that way. Very practical.

But charging through an USB port is only possible at one side of the USB connection, not on both sides. Basically, the USB ports on a laptop are used to send a signal out, thus they can be used to power up something. Many rechargable USB devices are designed to get a signal in, thus they are waiting until they receive power through the port. Technically, this is all just a one-direction street.

Still, they could make a laptop that can be recharged through the USB port, if they add a receiving end to it. Basically, that would mean that you can't connect a device to your laptop, but you would be connecting your laptop, as if it's just another device, to another PC. Would this be practical? Maybe if you want to do this to set up a special network or whatever but no one has seen some true value in such a setup. Basically, you'd be downgrading your laptop to a portable hard disk.

Still, as mentioned by the others, an average laptop will require a bit more power to fully charge than other devices. Especially the big screen and the disks are big energy consumers, although the CPU itself also puts some strain on the system. The power a device receives through the USB port is just a fraction of the power that a device would receive through a direct power connection.

The closest thing to a computer that can be recharged through an USB port would be a Smartphone with Windows Mobile. Or perhaps an UMPC system, but I don't know any good UMPC that is recharged this way. I assume you also have a few wishes concerning operating system, diskspace and memory.

I'm aware of no computer that can be charged using the USB port. The capacity of a USB connection is only up to 500 mA (milli-Amperes) at 5 volts, or 2.5 Watts per powered port. This is not sufficient to actually run a typical laptop these days. Its worthy to note that USB1.1 ports only 100mA.

this is actually not true (capacity of USB being 100mA). you're correct that USB supplies 5V. USB1 only supplied 100mA per port. USB2 supplies current in 100mA increments, up to a maximum of 500mA per port. the battery charging spec increases this to 1.8A if the device is only charging and not communicating (tho i don't know if that spec is part of USB2 or USB3). see en.wikipedia.org/wiki/Universal_Serial_Bus#Power
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quack quixoteJan 11 '10 at 6:17

Fair enough. However, that still limits one port to 9w, or not enough to run even a relatively low-power laptop. And no laptop still seems to support USB charging.
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CarlFJan 11 '10 at 6:44

true, but irrelevant; the question's about charging via usb, not running. theoretically you could trickle-charge a laptop battery over USB, given the circuitry to support that. i don't know of a product that actually supplies that, however. (mostly i was just posting a correction to your numbers; i probably should've just edited that in to begin with...)
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quack quixoteJan 11 '10 at 6:54

While this probably is probably not what you're looking for the HP Chromebook 11in that was released a few months (and recalled recently) charged using a mirco usb port. Its apparently will happily run off a 3 amp usb charger. There isn't a real physical limitation to usb, just like there isn't a real physical limitation to using 3.5mm aux jack to charge a small device (like the iPod Shuffle), but generally its a very bad idea to transfer charge in a direction its not expected to go.

This charger charges Notebooks by usual 19 V and a dedicated charging connector. It only has an additional charging/powering USB port as conventional USB chargers/power supplies. So this does not answer the question at all. See the User Guide. I am flagging the answer as low quality.
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paboukDec 3 '13 at 8:50